Nickel base alloy

ABSTRACT

Nickel base alloys containing tungsten, aluminum, zirconium, and carbon in preferred weight percentage ranges have superior strength in the 2,000*-2,200* F. temperature range. This alloy has a nominal composition of 20% tungsten, 6.5% aluminum, 1% zirconium, 0.2% carbon and the balance nickel.

United States Patent John C. Freche Fairview Park;

William J. Waters, Cleveland, both of Ohio 68,024

Aug. 28, 1970 Nov. 16, 1971 The United States of America as represented by the Administrator of the National Aeronautics and Space Administration [72] Inventors [21 Appl. No. [22] Filed [45] Patented [73] Assignee [54] NICKEL BASE ALLOY 5 Claims, No Drawings [52] US. Cl 75/170, 148/325 [51] Int. Cl C22c 19/00 [50] Fieldofsearch l7l', 148/32, 32.5

[56] References Cited UNITED STATES PATENTS 2,097,178 l0/l937 Golyer 75/170 3,188,204 6/1965 Bishop et al. 75/170 Primary Examiner-Richard 0. Dean AttorneysN T. Musial, G. E. Shook and John R. Manning ABSTRACT: Nickel base alloys containing tungsten, aluminum, zirconium, and carbon in preferred weight percentage ranges have superior strength in the 2,000-2,200 F. temperature range. This alloy has a nominal composition of 20% tungsten, 6.5% aluminum, 1% zirconium, 02% carbon and the balance nickel.

NICKEL BASE ALLOY ORIGIN OF THE INVENTION The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention is concerned with nickel base alloys which have superior strengths at elevated temperatures, high incipient melting points, high impact resistance, and are not subject to embrittlement upon long time exposure at intermediate temperatures. The invention is particularly directed to nickel base alloys having a low number of alloying constituents and which achieved these properties.

Higher turbine inlet temperatures such as those above 2,000 F. are necessary to meet the increased performance requirements of advanced gas turbine engines. This creates a need for materials that have improved strengths at high temperature. The stator vanes are subject to the highest gas temperatures of all the hot components of turbine engines. Nickel base alloys presently used for stator vane applications are extremely complex and contain many alloying constituents to achieve adequate high temperature strength. These alloys depend primarily on the gamma prime phase for the high temperature strength. Above l,900-2,000 F. temperature range the gamma prime phase agglomerates and the strength of these alloys decreases sharply.

All of the highly alloyed, complex nickel base alloys are subject to incipient melting at approximately 2,200 F. During turbine operation localized regions of the vanes can exceed this temperature due to nonunifonn combustion gas profiles and localized melting can occur. This can result in regions of weakness which become the nuclei for failure.

High strength nickel base alloys in use today are metallurgically very complex and can contain as many as or more alloying constituents. Generally speaking, the larger the number of alloying constituents the more the melting point of the resulting alloy is depressed. Therefore, it is desirable to drastically reduce the number of alloying constituents and to make the principal alloying constituent one that would tend to raise rather than lower the alloy melting point.

Some of the highly alloyed commercial base alloys are also subject to the formation of embrittling phases upon longtime exposure to intermediate temperatures. Such embrittlement can make engine components fabricated from these materials fail due to fatigue. It also can make them particularly susceptible to impact damage from the foreign objects that pass through the engine. The stator vanes are exposed to such intermediate temperatures during normal engine operation and considerable time can be accumulated at these temperatures.

SUMMARY OF THE INVENTION The need for materials having improved strength at high temperatures, high incipient melting points, microstructural stability, and high impact resistance has been met by the nickel base alloys of the present invention. The nominal compositions, in weight percentages, of these alloys are 17-22percent tungsten, 5.5-7.5 percent aluminum, 0.7-1.7 percent zirconium, 0.1-0.3 percent carbon, and the balance nickel. A preferred alloy of the invention has the nominal composition and weight percent of 17-20 percent tungsten, 6-7 percent aluminum, 1.4-1.6 percent zirconium, 0.1-0.2 percent carbon, and the balance nickel.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide an improved nickel base alloy having high tensile strength at 2,200 F.

Another object of the invention is to provide a nickel base alloy with a significantly higher incipient melting temperature than presently available, highly alloyed nickel base alloys.

A further object of the invention is to provide an improved nickel base alloy having higher impact strength than conventional nickel base alloys both before and after exposure to temperatures at which many nickel base alloys are subject to embrittlement.

Still another object of the invention is to provide an improved nickel base alloy that is not subject to the formation of embrittling phases after longtime exposures at intermediate temperatures.

A still further object of the invention is to provide an alloy which, upon application of directional solidification techniques, shows increased strength and ductility over its conventionally cast counterpart.

An additional object of the invention is to provide an improved nickel base alloy which can be rolled into a sheet with conventional rolling mill equipment using specialized techniques.

These and other objects of the invention will be apparent from the specification that follows.

DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention is embodied in alloys having the following composition range, the amount of each alloying element listed as a percentage by weight:

Percent Tungsten From about l7 to about 22 Aluminum From about 5.5 to about 7.5 Zirconium From about 0.7 to about 1.?

Carbon From about 0.] to about 0 3 Nickel Balance A preferred alloy has the following composition range by weight:

Tungsten From about 17% to'abou! 20% Aluminum From about 6% to about 7% Zirconium From about l.4% to about lbi Carbon From about 0.1% to about 0.2% Nickel Balance The high percentage of tungsten in the alloys tends to increase the temperature at which melting occurs. The tungsten also provides solid solution strength. Tungsten and zirconium contribute to the strength of the Ni Al (gamma prime) intermetallic phase, and contribute to the formation of stable carbides.

The aluminum present in the alloys also contributes to the formation of the Ni A1 intermetallic compound phase and contributes to high temperature oxidation resistance. The carbon enables stable carbides to be formed which can provide strength by preventing grain boundary sliding under load at high temperature.

Melts of the alloys were prepared in 50 kilowatt, 10,000 hertz, water-cooled induction units. Both double and triple melts were made. In all cases the initial melt was made under an inert gas cover of commercially pure argon is stabilized zirconia crucibles. The average exposure time per melt between melt and crucible was approximately 20 minutes. Carbon and tungsten additions were made in the form of powders precharged into the cold crucible with nickel platelets. Aluminum was added in the form of granules after the initial charge had melted.

The melt was subsequently superheated to approximately 3,000 F. and poured at 2,900 F. The melts were poured into copper chill molds and were cooled to room temperature to provide pigs for subsequent remelting. The second stage of the melting process was to remelt under vacuum. A pressure of 10 torr or less was maintained during melting and pouring. The molten alloy was superheated to approximately 3,000" F. prior to pouring. The pouring temperature for the random polycrystalline form of the alloy was 2,850fl5 F. Zircon shell molds preheated to l,600 F. were used to obtain test bars. In some cases the melts were again poured into copper molds to provide pigs for the third stage of the melting process.

The third stage of the melting process was done under vacuum and was employed only to make directionally solidified castings of the alloys. A three zone heater mold was utilized to provide a smooth temperature gradient along the length of the mold. The grain growth was initiated from a water-cooled copper chill block inserted at the base of the mold. The molten alloys were superheated to approximately 3,000 F. prior to pouring. The pour temperature was 2,850:25 F. The temperature of the portion of the mold adjacent to the chill block was approximately 2,450 F. After pouring, power to the mold heater zones was sequentially removed, and solidification proceeded vertically upward from the chill block.

The specimens were vapor blasted and inspected by X-ray and fluorescent-dye penetrant techniques before testing. Only defect-free bars were tested. Table 1 shows the compositions of selected heats of the alloy in both the random and directional polycrystalline form.

TABLE 1.ALLOY COMPOSITION Composition, wt. percent As set forth above, the random polycrystalline form of the alloy was obtained by argon induction melting followed by a single vacuum induction melt. The directional polycrystalline form was obtained by argon induction melting followed by two vacuum induction remelts.

Zirconium and trace amounts of other elements were picked up from the crucible during induction melting.

in the directionally oriented columnar grain structure the grains are grown parallel to the major stress axis. Substantial increases in intermediate temperature tensile strength, generally improved ductility, and increased stress rupture life compared to the random polycrystalline form of the alloy were realized. By way of example, at a stress of 15,000 p.s.i., the 100 hour use temperatures" are l,945 F. with the 4 directional polycrystalline form of the preferred alloy and 1 ,900 F. for the random polycrystalline form.

The alloys exhibit good strength characteristics at very high temperatures. By way of example, at 2,200? F. the preferred alloy has an ultimate tensile strength of 20,000 p.s.i. which is about two to three times that of other conventional, highly alloyed nickel base alloys. It is even higher than that of TD- Nickel, a dispersion strengthened powder metallurgy product.

The alloys have excellent room temperature impact strengths, both as cast and after long time exposure at l,600 F. Average Charpy notched impact strength for the as-cast alloys' were 10 and 16 foot-pounds in the random and directional polycrystalline forms, respectively. After 1,000 hours exposure at l,600 F. they were 16 and 20 foot-pounds, respectively, indicating that embrittlement did not occur during such exposure. These values are substantially higher than other known nickel base alloys.

The preferred alloy has an incipient melting temperature of approximately 2,375 F., which is l50-l75 F. higher than those of the strongest, conventional, highly alloyed nickel base alloys available today.

Although the present invention has been described in conjunction with the preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the spirit of the invention or the scope of the subjoined claims.

What is claimed: 1. A nickel base alloy consisting essentially of from l7-22 percent tungsten, from 5.5-7.5 percent aluminum, from 0.7-1.7 percent zirconium, from 0.1-0.3 percent carbon, and the balance nickel.

2. A nickel base alloy as claimed in claim 1 consisting essenr tially of from 17-20 percent tungsten, from 6-7 percent aluminum, from 1.4-1.6 percent zirconium, from 0.1-0.2 percent carbomand the balance nickel.

3. A random polycrystalline nickel base alloy as claimed in claim 1 consisting essentially of from l8.8-l9.7 percent tungsten, from 5.9-6.2 percent aluminum, from 0.8-l.4 percent zirconium, from 0.1-0.2 percent carbon, and the balance nickel.

4. A directional polycrystalline nickel base alloy as claimed in claim 1 consisting essentially of from l7.0-l7.9 percent tungsten, from 5.8-6.0 percent aluminum, from 1.5-1.7 percent zirconium, about 0.1 l percent carbon, and the balance nickel.

5. A nickel base alloy as claimed in claim 1 consisting essentially of 20 percent tungsten, 6.5 percent aluminum, 1 percent zirconium, 0.2 percent carbon, and the balance nickel. 

2. A nickel base alloy as claimed in claim 1 consisting essentially of from 17-20 percent tungsten, from 6-7 percent aluminum, from 1.4-1.6 percent zirconium, from 0.1-0.2 percent carbon, and the balance nickel.
 3. A random polycrystalline nickel base alloy as claimed in claim 1 consisting essentially of from 18.8-19.7 percent tungsten, from 5.9-6.2 percent aluminum, from 0.8-1.4 percent zirconium, from 0.1-0.2 percent carbon, and the balance nickel.
 4. A directional polycrystalline nickel base alloy as claimed in claim 1 consisting essentially of from 17.0-17.9 percent tungsten, from 5.8-6.0 percent aluminum, from 1.5-1.7 percent zirconium, about 0.11 percent carbon, and the balance nickel.
 5. A nickel base alloy as claimed in claim 1 consisting essentially of 20 percent tungsten, 6.5 percent aluminum, 1 percent zirconium, 0.2 percent carbon, and the balance nickel. 